Generally the prior art reports that the transesterification of aliphatic hydroxy compounds with carbonic acid, aliphatic diesters and aromatic diesters occurs readily in the presence of a basic catalyst and is a convenient method of synthesis of higher carbonates.
Several references deal with the transesterification of glycol carbonates using an aliphatic alcohol. Most demonstrate the use of methanol and ethylene carbonate.
There is taught in U.S. Pat. No. 3,803,201 a process for making dimethyl carbonate by methanolysis of alkylene carbonate wherein the improvement comprises removing the dimethyl carbonate from the reaction mixture during the reaction by distilling a mixture of methanol and dimethyl carbonate from the reaction mixture.
U.S. Pat. No. 4,307,032 discloses a process for the preparation of a dialkylcarbonate by contacting a glycol carbonate of a 1,2-diol with 2 to 4 carbon atoms with a selected alcohol to form the corresponding carbonate of said alcohol at a temperature of between 50 and 250.degree. C., in the presence of an improved catalyst which is a thallium compound, so that the reaction can take place under milder conditions. Thallium is however expensive and very toxic.
In another process disclosed in U.S. Pat. No. 4,181,676 there is taught a method for preparation of dialkyl carbonate by contacting a glycol carbonate of a 1,2-diol having 2 to 4 carbon atoms with a selected group of alcohols at an elevated temperature in the presence of an alkali metal or alkali metal compound wherein the improvement comprises employing less than 0.01 percent by weight of alkali metal or alkali metal compound based on the weight of the reaction mixture.
It is known that alkyl carbonates of the type ROCOOR can be obtained from alcohols and cyclic carbonates corresponding to the above formula through a transesterification reaction in the presence of alkali alcoholates or hydrates; however, moderate amounts of inorganic compounds are produced by these reactions which must be removed by methods which may unfavorably affect the general economy of the process.
In U.S. Pat. No. 4,062,884 this problem was addressed and it was found that dialkylcarbonates can be prepared by reacting alcohols with cyclic carbonates in the presence of organic bases, which makes it unnecessary to remove inorganic compounds and allows the catalyst to be totally recovered by means of simple distillation. The preferred organic base is a tertiary aliphatic amine.
U.S. Pat. No. 4,349,486 teaches a monocarbonate transesterification process comprising contacting a beta-fluoroaliphatic carbonate, a compound selected from the class of monohydroxy aliphatic alcohols, monohydroxy phenols and ortho-positioned dihyroxy aromatic compounds in the presence of a base. This invention claims to greatly reduce undesirable side reactions and only small amounts of carbonic acid-aliphatic-aromatic mixed diester is associated with the isolated aromatic monocarbonate reaction.
The Gilpin and Emmons Patent, referred to above, discusses problems associated with the separation of the methanol, dimethyl carbonate azeotrope and teaches one solution, wherein dimethyl carbonate is isolated from the azeotrope by a combination of low temperature crystallization and fractional distillation.
In the art there are also discussions of the transesterification reaction and the general acid-base catalysis of such systems. For example, in J. Am. Chem. Soc. 96(a) 2924-9 (1974), Hine and Kluppel discuss enthalpies for the reactions of esters, such as trimethyl orthoformate, triethyl orthoformate, tetramethyl orthocarbonate etc. with excess 65% tetrahydrofuran-35% water in the presence of acid at 25.degree. to give the corresponding simple esters and methanol or ethanol. Enthalpies of formation are calculated. It is found that the marked stabilization that accompanies the attachment of several alkoxy groups to the same saturated carbon atom may be illustrated by the disproportionation of dimethyl ether to tetramethyl orthocarbonate and methane.
In another article in the J. Org. Chem. 49(b) 1122-1125 (1984) Cella and Bacon discuss the results of their work. Among other things, they found that the alkylation of alkali metal bicarbonate and carbonate salts with alkyl halides in dipolar aprotic solvents and phase-transfer catalysts produces alkyl carbonates in good yields. The major limitation of this method is the failure of activated aryl halides or electronegatively substituted alkyl halides to produce carbonates due to the facility with which the intermediate alkoxy carbonate salts decompose.
Disadvantages of the methods discussed above include in many cases the fact that it is necessary to use a large amount of methanol feedstock relative to the amount of dimethyl carbonate produced. Also, in many cases alkali metal halides are coproduced and these present disposal problems.
It would be a substantial advance in the art to devise an efficient process for co-producing dimethyl carbonate and ethylene glycol which required only ca. 2-5 moles of methanol per mole of dimethyl carbonate produced. The dimethyl carbonate produced by this novel process could be used as a gasoline extender.